📚 Excretion in Humans: Kidney Function and Osmoregulation | 人体排泄：肾脏功能与渗透调节
Excretion is the removal from the body of the waste products of metabolism, which are produced by chemical reactions inside cells. In humans, excretion is carried out mainly by the lungs, the liver, the skin and the kidneys, and the kidneys play the central role in removing nitrogenous waste and in regulating the water and ion balance of the body. This article covers the definition of excretion, deamination and urea formation in the liver, the structure of the nephron, ultrafiltration, selective reabsorption, the loop of Henle, ADH and osmoregulation, and the clinical consequences when the kidneys fail.
排泄是指将代谢废物从体内清除的过程，这些废物由细胞内的化学反应产生。在人体中，排泄主要由肺、肝脏、皮肤和肾脏完成，其中肾脏在清除含氮废物以及调节体内水和离子平衡方面发挥核心作用。本文将依次讲解排泄的定义、肝脏中的脱氨基作用与尿素生成、肾单位的结构、超滤作用、选择性重吸收、髓袢、ADH 与渗透调节，以及肾功能衰竭带来的后果。
1. What is Excretion? | 什么是排泄？
Excretion is the removal from the body of the waste products of metabolism, that is, substances that have been made inside cells by chemical reactions. It is a homeostatic process. Most metabolic waste products are toxic, and if they were allowed to accumulate they would inhibit enzymes, damage cells and disturb the chemical composition of the internal environment. Typical excretory products in humans include carbon dioxide from aerobic respiration, urea from the breakdown of excess amino acids, bile pigments from the breakdown of haemoglobin, and water and ions taken in beyond the requirements of the body.
排泄是指将代谢产生的废物从体内清除，也就是把细胞内的化学反应所生成的物质排出体外。它是一个维持稳态的过程。大多数代谢废物具有毒性，如果任其积累，就会抑制酶的活性、损伤细胞并扰乱内环境的化学组成。人体典型的排泄产物包括有氧呼吸产生的二氧化碳、过量氨基酸分解产生的尿素、血红蛋白分解产生的胆色素，以及摄入量超过身体需求的水和离子。
The waste products of metabolism fall into two broad groups: gaseous waste such as carbon dioxide, which is removed by the lungs, and soluble nitrogenous waste such as urea, which is removed by the kidneys and, to a much smaller extent, in sweat. Water and inorganic ions are not strictly waste in the same sense, but any surplus must be excreted so that the water potential and the ionic composition of the tissue fluid stay within narrow limits. The kidneys therefore act as both excretory organs and regulators of the internal environment.
代谢废物大致可分为两类：一类是二氧化碳等气态废物，由肺排出；另一类是尿素等可溶性的含氮废物，主要由肾脏排出，小部分随汗液排出。水和无机离子严格来说并不属于同一意义上的废物，但任何多余的部分都必须被排出，这样才能使组织液的水势和离子组成保持在很窄的范围内。因此，肾脏既是排泄器官，也是内环境的调节器官。
metabolism → waste products → excretion → stable internal environment
2. Excretion versus Egestion | 排泄与排遗的区别
Excretion must not be confused with egestion. Egestion is the removal of undigested food material from the gut as faeces. That material has never been absorbed into the cells of the body, so it is not a product of metabolism. Excretion, by contrast, deals only with substances that have been produced inside cells by metabolic reactions, such as urea, carbon dioxide and bile pigments. A useful test in an exam is to ask whether the substance was made inside the body's cells: if it was, its removal is excretion.
不能把排泄与排遗混为一谈。排遗是指将消化道中未被消化的食物残渣以粪便的形式排出体外。这些物质从未被吸收进入体内的细胞，因此不是代谢产物。相反，排泄只处理细胞内由代谢反应生成的物质，例如尿素、二氧化碳和胆色素。考试中一个实用的判断方法是问：这种物质是否是在人体细胞内生成的？如果是，那么它的排出就属于排泄。
| Feature | 特征 | Excretion | 排泄 | Egestion | 排遗 |
| Source of material | 物质来源 | Made inside cells by metabolism | 细胞内代谢产生 | Unabsorbed food in the gut | 消化道内未吸收的食物 |
| Typical examples | 典型例子 | Urea, CO₂, bile pigments | 尿素、二氧化碳、胆色素 | Faeces, cellulose, undigested protein | 粪便、纤维素、未消化的蛋白质 |
| Main route | 主要途径 | Kidneys, lungs, skin, liver | 肾脏、肺、皮肤、肝脏 | Anus via the colon and rectum | 经结肠和直肠由肛门排出 |
| Release of energy | 能量释放 | Products of catabolism | 分解代谢的产物 | Never entered metabolism | 从未参与代谢 |
3. The Organs of Excretion | 排泄器官
The lungs excrete carbon dioxide and water vapour. Carbon dioxide is produced by aerobic respiration in every living cell and is transported to the lungs in the blood, mainly as hydrogencarbonate ions (HCO₃⁻). The liver is an excretory organ because it produces urea by deamination, breaks down haemoglobin to form bile pigments that are excreted in bile, and detoxifies a wide range of harmful substances. The skin excretes a small amount of water, salts, urea and lactic acid in sweat, although the main function of sweating is thermoregulation rather than excretion.
肺排出二氧化碳和水蒸气。二氧化碳由每一个活细胞的有氧呼吸产生，主要以碳酸氢根离子（HCO₃⁻）的形式随血液运输到肺。肝脏也是排泄器官，因为它通过脱氨基作用生成尿素，将血红蛋白分解形成随胆汁排出的胆色素，并对多种有害物质进行解毒。皮肤通过汗液排出少量的水、盐、尿素和乳酸，不过出汗的主要功能是体温调节，而不是排泄。
The kidneys are the principal excretory organs in humans. Each kidney receives blood through the renal artery and returns it through the renal vein, and urine leaves the kidney through the ureter. The kidneys remove urea, creatinine, excess water, excess ions and certain drugs and hormones. They also regulate the pH of the blood by excreting H⁺ ions and reabsorbing HCO₃⁻ ions, and they help to control blood pressure through the renin-angiotensin mechanism, which adjusts sodium reabsorption and blood volume.
肾脏是人体的主要排泄器官。每个肾脏通过肾动脉接受血液，再通过肾静脉将血液送回，尿液则经输尿管离开肾脏。肾脏清除尿素、肌酐、多余的水、多余的离子以及某些药物和激素。肾脏还通过排出 H⁺ 离子、重吸收 HCO₃⁻ 离子来调节血液的 pH，并通过肾素-血管紧张素机制调节钠的重吸收和血容量，从而帮助控制血压。
4. Nitrogenous Waste and Deamination in the Liver | 含氮废物与肝脏中的脱氨基作用
Amino acids produced by the digestion of protein are absorbed into the blood and used to build new body proteins such as enzymes, haemoglobin and antibodies. However, the body has no store for amino acids, and any amino acid that is not needed for protein synthesis cannot simply be kept. Instead it is transported to the liver, where it is broken down. The crucial first step is deamination, in which the amino group (-NH₂) is removed from the amino acid.
蛋白质被消化后产生的氨基酸被吸收进入血液，用于合成酶、血红蛋白和抗体等新的体内蛋白质。然而，人体没有储存氨基酸的机制，任何不需要用于蛋白质合成的氨基酸都不能简单地留存下来。它们会被运送到肝脏并在那里分解。关键的第一步就是脱氨基作用，即从氨基酸上去除氨基（-NH₂）。
Deamination produces two products: ammonia (NH₃) and a ketone acid, sometimes called a keto acid. The ammonia is very toxic and extremely soluble in water, so it cannot be allowed to circulate in the blood in large amounts and must be dealt with immediately. The keto acid is not a waste product at all. It has a useful carbon skeleton that can be respired to release energy, converted to carbohydrate and stored as glycogen, or converted to fat and stored in adipose tissue. In this way the body rescues the energy value of the amino acid while disposing of the nitrogen safely.
脱氨基作用生成两种产物：氨（NH₃）和一种酮酸。氨毒性很强且极易溶于水，因此不能让其大量在血液中循环，必须立即处理。而酮酸根本不是废物：它拥有有用的碳骨架，可以被呼吸作用氧化释放能量，也可以转化为碳水化合物并以糖原形式储存，或者转化为脂肪储存在脂肪组织中。这样，人体在安全处理氮的同时，保留了氨基酸的能量价值。
amino acid → keto acid + NH₃ (deamination in the liver)
5. From Ammonia to Urea | 从氨到尿素
Because ammonia is so toxic, land animals convert it into a less harmful compound before it is transported and excreted. In the liver, ammonia is combined with carbon dioxide in a series of reactions sometimes called the ornithine cycle. The overall result is urea, (NH₂)₂CO, which is far less toxic than ammonia and much less soluble in water. Urea can therefore be carried safely in the bloodstream at relatively high concentrations and excreted in a concentrated, small volume of urine, which conserves water.
由于氨毒性很强，陆生动物会在运输和排出之前把它转化为危害较小的化合物。在肝脏中，氨与二氧化碳经过一系列反应（有时称为鸟氨酸循环）结合，总的结果是生成尿素 (NH₂)₂CO。尿素的毒性远低于氨，在水中的溶解度也小得多。因此尿素可以以相对较高的浓度安全地在血液中运输，并以体积很小的浓缩尿液排出，从而节约水分。
Different animals excrete nitrogenous waste in different forms, and the form used reflects the availability of water in the habitat. Bony fish that live in fresh water excrete ammonia directly across their gills, because the ammonia can be diluted at once by a large volume of surrounding water. Birds, reptiles and insects excrete uric acid, which is almost insoluble and is released as a white paste together with the faeces, so very little water is lost. Humans and other mammals excrete urea, which is a compromise between toxicity and the need to conserve water.
不同动物排出含氮废物的形式不同，所采用的形式反映了其栖息地中水分的可获得性。生活在淡水中的硬骨鱼直接通过鳃排出氨，因为周围的巨量水体可以立刻将氨稀释。鸟类、爬行动物和昆虫排出尿酸，尿酸几乎不溶于水，以白色糊状物随粪便一起排出，因此水分损失极少。人类和其他哺乳动物排出尿素，这是在毒性与节约水分需求之间的折中方案。
| Waste | 废物 | Toxicity | 毒性 | Water needed | 需水量 | Example | 例子 |
| Ammonia NH₃ | Very high | 很高 | Very large | 非常大 | Freshwater fish | 淡水鱼 |
| Urea (NH₂)₂CO | Moderate | 中等 | Moderate | 中等 | Mammals, adult amphibians | 哺乳动物、成体两栖类 |
| Uric acid | Low | 低 | Very small | 非常小 | Birds, reptiles, insects | 鸟类、爬行类、昆虫 |
6. Structure of the Kidney and the Nephron | 肾脏与肾单位的结构
Each human kidney is a bean-shaped organ about 11 cm long. A longitudinal section shows three regions: the cortex on the outside, the medulla in the middle with its pyramids, and the pelvis, a funnel-shaped cavity that collects urine and leads into the ureter. Blood enters through the renal artery, which branches into smaller and smaller arterioles, and leaves through the renal vein. Urine passes from the pelvis down the ureter to the bladder, where it is stored, and is then expelled through the urethra.
人的每个肾脏是一个长约 11 厘米的豆形器官。纵切面可见三个区域：外侧的皮质、中间带有锥体的髓质，以及漏斗状的肾盂，肾盂收集尿液并通向输尿管。血液经肾动脉进入，肾动脉不断分支成越来越细的小动脉，血液再经肾静脉离开。尿液从肾盂沿输尿管流到膀胱储存，随后经尿道排出。
The functional unit of the kidney is the nephron, and each kidney contains about one million of them. A nephron consists of a Bowman's capsule surrounding a ball of capillaries called the glomerulus, followed by the proximal convoluted tubule in the cortex, the loop of Henle which dips into the medulla, the distal convoluted tubule back in the cortex, and finally the collecting duct, which runs down through the medulla and empties into the pelvis. The blood supply is arranged so that a dense capillary network, the vasa recta, runs alongside the loop of Henle.
肾脏的功能单位是肾单位，每个肾脏大约含有一百万个肾单位。一个肾单位由包裹着毛细血管球（肾小球）的肾小囊构成，其后依次是位于皮质中的近曲小管、伸入髓质的髓袢、回到皮质中的远曲小管，最后是穿过髓质、开口于肾盂的集合管。血液供应被安排成密集的毛细血管网，即直小血管，与髓袢平行走行。
7. Ultrafiltration in the Glomerulus | 肾小球的超滤作用
Ultrafiltration is the filtration of blood under pressure through the walls of the glomerulus and the inner wall of the Bowman's capsule. The afferent arteriole bringing blood into the glomerulus is wider than the efferent arteriole taking blood out, so the blood in the glomerular capillaries is under a high hydrostatic pressure. This pressure forces water and small dissolved molecules out of the blood plasma and into the capsule, forming the glomerular filtrate.
超滤作用是指血液在压力下通过肾小球壁和肾小囊内壁被过滤的过程。将血液送入肾小球的入球小动脉比将血液送出的出球小动脉更粗，因此肾小球毛细血管内的血液承受很高的静水压。这一压力把水和小的溶质分子从血浆中压出，进入肾小囊，形成肾小球滤液。
The filter itself is selective because of the basement membrane and the cells of the capsule. The basement membrane acts as a molecular sieve: water, glucose, amino acids, salts, urea and other small molecules pass through, but large plasma proteins and blood cells are held back. The filtrate therefore contains no proteins and no cells, which is why healthy urine does not froth and does not contain blood. The rate at which filtrate is formed is called the glomerular filtration rate, and in a healthy adult it is around 125 cm³ per minute.
由于基膜和肾小囊细胞的存在，这一过滤器具有选择性。基膜起分子筛的作用：水、葡萄糖、氨基酸、盐、尿素等小分子可以通过，而大分子的血浆蛋白和血细胞则被阻挡。因此滤液中不含蛋白质和细胞，这也是健康人的尿液不起泡沫、不含血液的原因。滤液生成的速率称为肾小球滤过率，健康成年人约为每分钟 125 cm³。
blood plasma + high pressure → glomerular filtrate (no proteins, no cells)
8. Selective Reabsorption in the Proximal Convoluted Tubule | 近曲小管的选择性重吸收
If the glomerular filtrate were simply excreted, the body would lose about 180 dm³ of water every day, together with all of its glucose, amino acids and salts. Selective reabsorption prevents this. As the filtrate flows along the proximal convoluted tubule, useful substances are taken back into the blood. The cells of this tubule are adapted for the task: their surface facing the lumen has many microvilli to give a large surface area, and they contain large numbers of mitochondria to supply ATP for active transport.
如果肾小球滤液被直接排出，人体每天将损失约 180 dm³ 的水，以及全部葡萄糖、氨基酸和盐。选择性重吸收可以避免这种损失。当滤液流经近曲小管时，有用物质被重新吸收回血液。该小管的细胞为此高度适应：面向管腔的一侧有许多微绒毛以增大表面积，细胞中还含有大量线粒体，为主动运输提供 ATP。
About 85 per cent of the water in the filtrate is reabsorbed here by osmosis, together with all of the glucose and amino acids and most of the sodium and chloride ions. Sodium ions are actively pumped out of the tubule cells into the blood, creating a gradient that drives the co-transport of glucose and amino acids back into the cells. Reabsorption of these solutes lowers the water potential of the blood and raises that of the filtrate, so water follows by osmosis. Some urea is also reabsorbed passively, which is why the concentration of urea in urine is high but not as high as it would be if no urea were recovered at all.
约 85% 的滤液中的水在此处通过渗透作用被重吸收，同时全部葡萄糖和氨基酸以及大部分钠离子和氯离子也被重吸收。钠离子被主动泵出小管细胞进入血液，所形成的梯度驱动葡萄糖和氨基酸经协同运输回到细胞内。这些溶质的重吸收降低了血液的水势，同时提高了滤液的水势，于是水便通过渗透作用跟随进入血液。部分尿素也被动地被重吸收，这就是尿液中尿素浓度很高、但并非完全不回收尿素时那样高的原因。
9. The Loop of Henle and the Countercurrent Multiplier | 髓袢与逆流倍增机制
The loop of Henle creates the very high solute concentration in the medulla that allows the kidneys to produce concentrated urine. Its two limbs have completely different properties. The descending limb is permeable to water but has very low permeability to salts, so as the filtrate descends into the medulla, water leaves by osmosis and the filtrate becomes more concentrated. The ascending limb is impermeable to water but has active sodium and chloride pumps, so as the filtrate ascends, salts are pumped out into the medullary tissue without any water following.
髓袢在髓质中建立起极高的溶质浓度，使肾脏能够产生浓缩尿。它的两段具有完全不同的性质。降支对水通透，但对盐的通透性极低，因此当滤液向下进入髓质时，水通过渗透作用离开，滤液变得越来越浓。升支对水不通透，但具有主动的钠、氯泵，因此当滤液向上流动时，盐被泵出到髓质组织中，而水并不会随之排出。
Because the two limbs are arranged as a hairpin and fluid flows in opposite directions, the effect is multiplied along the length of the loop. The result is a steep concentration gradient in the medulla, from about 300 arbitrary units in the cortex to more than 1200 near the tip of the loop. The collecting duct passes down through this salty medulla, and if its walls are permeable to water, water is drawn out by osmosis and reabsorbed into the blood, leaving behind a small volume of highly concentrated urine. If the walls are impermeable to water, dilute urine is produced instead. The permeability of the collecting duct is controlled by ADH.
由于两段呈发夹状排列且液体流动方向相反，这种效应沿髓袢长度被不断放大。其结果是髓质中形成陡峭的浓度梯度，从皮质中约 300 个任意单位升高到髓袢顶端附近的 1200 以上。集合管穿过这片高盐
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